linux-kernel - Re: [PATCH net-next v9 2/3] net: gro: move L3 flush checks to tcp_gro_receive and udp_gro_receive

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Date: Thu, 9 May 2024 21:02:37 +0200
From: Richard Gobert <richardbgobert@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: alexander.duyck@...il.com, davem@...emloft.net, dsahern@...nel.org,
 edumazet@...gle.com, kuba@...nel.org, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org, netdev@...r.kernel.org, pabeni@...hat.com,
 shuah@...nel.org
Subject: Re: [PATCH net-next v9 2/3] net: gro: move L3 flush checks to
 tcp_gro_receive and udp_gro_receive_segment

Willem de Bruijn wrote:
> Richard Gobert wrote:
>> {inet,ipv6}_gro_receive functions perform flush checks (ttl, flags,
>> iph->id, ...) against all packets in a loop. These flush checks are used in
>> all merging UDP and TCP flows.
>>
>> These checks need to be done only once and only against the found p skb,
>> since they only affect flush and not same_flow.
>>
>> This patch leverages correct network header offsets from the cb for both
>> outer and inner network headers - allowing these checks to be done only
>> once, in tcp_gro_receive and udp_gro_receive_segment. As a result,
>> NAPI_GRO_CB(p)->flush is not used at all. In addition, flush_id checks are
>> more declarative and contained in inet_gro_flush, thus removing the need
>> for flush_id in napi_gro_cb.
>>
>> This results in less parsing code for non-loop flush tests for TCP and UDP
>> flows.
>>
>> To make sure results are not within noise range - I've made netfilter drop
>> all TCP packets, and measured CPU performance in GRO (in this case GRO is
>> responsible for about 50% of the CPU utilization).
>>
>> perf top while replaying 64 parallel IP/TCP streams merging in GRO:
>> (gro_receive_network_flush is compiled inline to tcp_gro_receive)
>> net-next:
>>         6.94% [kernel] [k] inet_gro_receive
>>         3.02% [kernel] [k] tcp_gro_receive
>>
>> patch applied:
>>         4.27% [kernel] [k] tcp_gro_receive
>>         4.22% [kernel] [k] inet_gro_receive
>>
>> perf top while replaying 64 parallel IP/IP/TCP streams merging in GRO (same
>> results for any encapsulation, in this case inet_gro_receive is top
>> offender in net-next)
>> net-next:
>>         10.09% [kernel] [k] inet_gro_receive
>>         2.08% [kernel] [k] tcp_gro_receive
>>
>> patch applied:
>>         6.97% [kernel] [k] inet_gro_receive
>>         3.68% [kernel] [k] tcp_gro_receive
>>
>> Signed-off-by: Richard Gobert <richardbgobert@...il.com>
> 
>> +static inline int inet_gro_flush(const struct iphdr *iph, const struct iphdr *iph2,
>> +				 struct sk_buff *p, bool outer)
>> +{
>> +	const u32 id = ntohl(*(__be32 *)&iph->id);
>> +	const u32 id2 = ntohl(*(__be32 *)&iph2->id);
>> +	const u16 ipid_offset = (id >> 16) - (id2 >> 16);
>> +	const u16 count = NAPI_GRO_CB(p)->count;
>> +	const u32 df = id & IP_DF;
>> +	int flush;
>> +
>> +	/* All fields must match except length and checksum. */
>> +	flush = (iph->ttl ^ iph2->ttl) | (iph->tos ^ iph2->tos) | (df ^ (id2 & IP_DF));
>> +
>> +	if (outer && df)
>> +		return flush;
> 
>     if (flush)
>             return 1;
> 
> To be able to avoid the two flush | below?
> Or to avoid adding a branch
> 
>     if (flush | (outer && df))
>             return 1;
> 
>> +
>> +	/* When we receive our second frame we can make a decision on if we
>> +	 * continue this flow as an atomic flow with a fixed ID or if we use
>> +	 * an incrementing ID.
>> +	 */
>> +	if (count == 1 && df && !ipid_offset)
>> +		NAPI_GRO_CB(p)->ip_fixedid = true;
>> +
>> +	if (NAPI_GRO_CB(p)->ip_fixedid && df)
>> +		return flush | ipid_offset;
>> +
>> +	return flush | (ipid_offset ^ count);
> 
> And then simply
> 
>     if (NAPI_GRO_CB(p)->ip_fixedid)
>             return ipid_offset;
>     else
>             return ipid_offset ^ count;
> 
> Since NAPI_GRO_CB(p)->ip_fixedid is only set if DF is set on the first
> two segments, and df ^ id2 & IP_DF is tested above, no need to test
> that again.
> 
>> +}

I like the idea, it is more readable.
We just need to return flush, and not 1 to make it correct (since flush
could be 0 while outer && df set):

if (flush | (outer && df))
		return flush;

Not setting NAPI_GRO_CB(p)->ip_fixedid when flush==1 is a slight change
from the previous behaviour.
AFAIU it is ok since it doesn't change GRO logic - p will be flushed from
gro_list and NAPI_GRO_CB becomes irrelevant.
Removing the DF check is nice, I also think we can avoid a branch
while keeping the code readable as follows:

return ipid_offset ^ (count * !NAPI_GRO_CB(p)->ip_fixedid);